15,962 research outputs found
Rubidium and zirconium abundances in massive Galactic asymptotic giant branch stars revisited
Luminous Galactic OH/IR stars have been identified as massive (>4-5 M_s) AGB
stars experiencing HBB and Li production. Their Rb abundances and [Rb/Zr]
ratios derived from hydrostatic model atmospheres, are significantly higher
than predictions from AGB nucleosynthesis models, posing a problem to our
understanding of AGB evolution and nucleosynthesis. We report new Rb and Zr
abundances in the full sample of massive Galactic AGB stars, previously studied
with hydrostatic models, by using more realistic extended model atmospheres. We
use a modified version of the spectral synthesis code Turbospectrum and
consider the presence of a circumstellar envelope and radial wind. The Rb and
Zr abundances are determined from the 7800 A Rb I resonant line and the 6474 A
ZrO bandhead, respectively, and we explore the sensitivity of the derived
abundances to variations of the stellar (Teff) and wind (M_loss, beta and vexp)
parameters in the extended models. The Rb and Zr abundances derived from the
best spectral fits are compared with the most recent AGB nucleosynthesis
theoretical models. The new Rb abundances are much lower (even 1-2 dex) than
those derived with the hydrostatic models, while the Zr abundances are similar.
The Rb I line profile and Rb abundance are very sensitive to the M_loss rate
but much less sensitive to variations of the wind velocity-law and the
vexp(OH). We confirm the earlier preliminary results based on a smaller sample
of massive O-rich AGB stars, that the use of extended atmosphere models can
solve the discrepancy between the AGB nucleosynthesis theoretical models and
the observations of Galactic massive AGB stars. The Rb abundances, however, are
still strongly dependent of the M_loss, which is unknown in these AGB stars.
Accurate M_loss rates in these massive Galactic AGB stars are needed in order
to break the models degeneracy and get reliable Rb abundances in these stars.Comment: Accepted for publication in A&A, 14 pages, 12 figures, 4 table
Exploring the structure of the quenched QCD vacuum with overlap fermions
Overlap fermions have an exact chiral symmetry on the lattice and are thus an
appropriate tool for investigating the chiral and topological structure of the
QCD vacuum. We study various chiral and topological aspects of quenched gauge
field configurations. This includes the localization and chiral properties of
the eigenmodes, the local structure of the ultraviolet filtered field strength
tensor, as well as the structure of topological charge fluctuations. We
conclude that the vacuum has a multifractal structure.Comment: 68 pages, 31 figures, file size: 1.7 MB (PDF
SU(2) Gluodynamics and HP1 sigma-model embedding: Scaling, Topology and Confinement
We investigate recently proposed HP1 sigma-model embedding method aimed to
study the topology of SU(2) gauge fields. The HP1 based topological charge is
shown to be fairly compatible with various known definitions. We study the
corresponding topological susceptibility and estimate its value in the
continuum limit. The geometrical clarity of HP1 approach allows to investigate
non-perturbative aspects of SU(2) gauge theory on qualitatively new level. In
particular, we obtain numerically precise estimation of gluon condensate and
its leading quadratic correction. Furthermore, we present clear evidences that
the string tension is to be associated with global (percolating) regions of
sign-coherent topological charge. As a byproduct of our analysis we estimate
the continuum value of quenched chiral condensate and the dimensionality of
regions, which localize the lowest eigenmodes of overlap Dirac operator.Comment: 22 pages, 18 ps figures, revtex4. Replaced to match published version
(PRD, to appear
Measurement-based quantum computation beyond the one-way model
We introduce novel schemes for quantum computing based on local measurements
on entangled resource states. This work elaborates on the framework established
in [Phys. Rev. Lett. 98, 220503 (2007), quant-ph/0609149]. Our method makes use
of tools from many-body physics - matrix product states, finitely correlated
states or projected entangled pairs states - to show how measurements on
entangled states can be viewed as processing quantum information. This work
hence constitutes an instance where a quantum information problem - how to
realize quantum computation - was approached using tools from many-body theory
and not vice versa. We give a more detailed description of the setting, and
present a large number of new examples. We find novel computational schemes,
which differ from the original one-way computer for example in the way the
randomness of measurement outcomes is handled. Also, schemes are presented
where the logical qubits are no longer strictly localized on the resource
state. Notably, we find a great flexibility in the properties of the universal
resource states: They may for example exhibit non-vanishing long-range
correlation functions or be locally arbitrarily close to a pure state. We
discuss variants of Kitaev's toric code states as universal resources, and
contrast this with situations where they can be efficiently classically
simulated. This framework opens up a way of thinking of tailoring resource
states to specific physical systems, such as cold atoms in optical lattices or
linear optical systems.Comment: 21 pages, 7 figure
Kaposi's sarcoma in a patient with erythroblastopenia and thymoma: Reactivation after topical corticosteroids
We report a 69-year-old female with erythroblastopenia and thymoma who developed lesions of Kaposi's sarcoma (KS) after thymectomy, 2 months after the initiation of therapy with methylprednisolone. Control of mucocutaneous KS lesions was obtained with radiotherapy, interferon alfa-2b and withdrawal of systemic immunosuppressive therapy. Erosive oral lichen planus appeared later, and after therapy with topical corticosteroids a new lesion of KS developed that regressed after withdrawal of topical corticosteroids. The detection of HHV-8 only in lesional skin supports the hypothesis that this virus can trigger the development of KS lesions
Calorons and dyons at the thermal phase transition analyzed by overlap fermions
In a pilot study, we use the topological charge density defined by the
eigenmodes of the overlap Dirac operator (with ultraviolet filtering by
mode-truncation) to search for lumps of topological charge in SU(2) pure gauge
theory. Augmenting this search with periodic and antiperiodic temporal boundary
conditions for the overlap fermions, we demonstrate that the lumps can be
classified either as calorons or as separate caloron constituents (dyons).
Inside the topological charge clusters the (smeared) Polyakov loop is found to
show the typical profile characteristic for calorons and dyons. This
investigation, motivated by recent caloron/dyon model studies, is performed at
the deconfinement phase transition for SU(2) gluodynamics on 20^3 x 6 lattices
described by the tadpole improved L\"uscher-Weisz action. The transition point
has been carefully located. As a necessary condition for the caloron/dyon
detection capability, we check that the LW action, in contrast to the Wilson
action, generates lattice ensembles, for which the overlap Dirac eigenvalue
spectrum smoothly behaves under smearing and under the change of the boundary
conditions.Comment: 17 pages, 8 figures, minor changes (typos, grants, ..
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